Passive heat transfer devices, such as heat pipes, are of much interest in applications such as electronics cooling. Heat pipes are a liquid and vapor device in which liquid is pumped through capillarity from the condenser to the evaporator. The pumping effect in this device requires a wick, which produces a high pressure loss and limits the maximum heat transport distance and or power that can be supported before dry-out occurs.
Another technology node that is useful is a thermosyphon as shown in FIG. 1. In operation, liquid 104 is vaporized in an evaporator 101. The vapor then travels through a tube 102 to the condenser 100. Heat is removed from the condenser 100 causing the liquid 104 to accumulate at the bottom. The accumulated liquid 104 in the condenser is driven by gravity through a liquid line 103 back to the evaporator 101. The evaporators in these devices are typically pool boiling devices with an enhanced surface 105 that may consist of fins, a porous layer or even an etched surface. The maximum boiling heat transfer coefficient can be limited in this device because there are a finite amount of nucleation sites, and therefore a limited length of solid/liquid/vapor contact, where the heat transfer rate is the highest.
In conventional thermosyphon design, a flow pattern that enters one side of the evaporator and leaves the other side, through a series of channels is typically not used. While this general concept is widely used in most heat transfer products, the implementation in thermosyphon design for electronics is generally prohibited by the limited pressure head provided by gravity to drive the flow and flow instabilities encountered with vapor expansion in a confined channel as shown in FIG. 2. As a channel size 201 decreases to the same size of a vapor bubble 202, the expansion of the vapor causes liquid 203 to flow outwards 204, irrespective of the desired flow rate. This phenomena poses a few problems. One problem is that the pressure drop associated with high liquid velocities in a channel are quite high, especially relative to the small available pressure head in a thermosyphon device. A second problem that this phenomena can cause is that the middle of the channel is left dry and can increase in temperature, since the vapor has limited heat capacitance.